Optical time domain reflectometers(OTDR) and methods for the test of optical components, such as optical fibers, are commonly used in optical communication systems. In practice there is a need for the characterization of an optical fiber's attenuation, uniformity, splice loss, breaks and length. In a known OTDR arrangement a pulse generator drives a laser diode which then launches optical pulses (10 mW or more) into an optical fiber to be tested. The pulse width ranges from nanoseconds to microseconds at repetition rates of 1 kHz (for long fiber lengths) up to 20 kHz (for short fiber lengths). The repetition rate is chosen such that the signals returning from the optical fiber do not overlap. The returning signal is separated from the launched signal by a directional coupler, such as a twisted-pair coupler or a polarizing beam splitter. Often an avalanche photodiode is used as a detector or OTDR receiver. Its signal is fed to an amplifier and a digitizer. A box car averager usually improves the signal-to-noise ratio. The signal is then displayed in logarithmic form. The weakness of the signals back-scattered by an optical component under test requires signal processing. In OTDRs this is commonly done in the digital data domain. Digital data is obtained by use of analog-to-digital converters (ADC) which convert analog signals into binary codes representing quantized amplitude values closest to the input value. As the conversion is not instantaneous, the output of an ADC is a discrete-time data sequence. Thus, theorems related to periodic sampling apply to the ADC output data as disclosed e.g. in A. V. Oppenheim, R. W. Schafer: Discrete-Time Signal Processing; Prentice Hall, New Jersey 1989.
As a consequence, in general the digital data is different from the true analog data. The amplitude difference is called quantization error. This quantization error leads to hard deterioration of measurement results in OTDRs and limits the lowest signal level which can be measured.
E.g. from F. Sischka, S. A. Newton, M. Nazarathy: Complementary Correlation Optical Time-Domain Reflectometry; Hewlett-Packard Journal, December 1988 it is known to add a dither signal to the analog input of the ADC to decrease the quantization error if a couple of individual measurements were taken and an average is calculated out of the digital results. After sufficient averaging is done, the well-known sawtooth error function of the ADC is smoothed to a sine function with drastically reduced amplitude. Such an averaging process implemented in all common OTDRs has two benefits. First, the quantization error is reduced, and second, the signal-to-noise ratio of the measurement result increases, as disclosed e.g. in J. Beller: A High-Performance Signal Processing System for the HP8146A Optical Time Domain Reflectometer; Hewlett-Packard Journal, February 1993.
Commonly, the electronic circuit of an OTDR receiver generates a certain amount of noise that is used in standard OTDRs to act as the dither signal already mentioned. An amplifier is used to scale the noise level at the ADC input to an appropriate level, with regard to the quantization step of the ADC. A high gain leads to a large noise amplitude which avoids a quantization error and results in improved linearity. However, as signal plus noise is amplified, high gain also limits the ADC's conversion range, i.e. clipping occurs at lower signal levels than it will be the case with lower gain. Hence, signal conversion of an ADC is limited by the maximum level on the upper side and the noise level on the lower side. Therefore, the noise level has an impact on both, the signal-to-noise ratio (dynamic range of measurement result) and linearity. This trade-off forces designers to a compromise regarding improved dynamic range and good linearity.
Thus, there is a need for an optical time domain reflectometer (OTDR) and a method for testing optical components, such as optical fibers, comprising a wide dynamic range and good linearity.